In the related art, an LN modulator is used in an optical modulation device that realizes a high-speed optical transmission system. The LN modulator modulates the light input from a luminous element such as an LD (Laser Diode) to output an optical signal.
Also, it is known that a modulation curve of the LN modulator is formed with a cos-squared function curve in a case where the vertical axis of a two-dimensional coordinate axis indicates the optical signal intensity (i.e. optical intensity) and the horizontal axis indicates a bias voltage. In an LN modulator adopting a CS-RZ (Carrier Suppression-Return to Zero) modulation scheme, an optical duobinary modulation scheme, a DPSK (Differential Phase Shift Keying) modulation scheme or a DQPSK (Differential Quadrature Phase-Shift Keying) modulation scheme, a bias voltage having an amplitude of 2 Vπ is used. Here, Vπ represents a voltage capable of changing a phase of the light input from the LN modulator by π.
In the LN modulator, as illustrated in FIG. 1, a DC drift occurs in which the modulation curve moves in the horizontal axis direction over time. Accordingly, an optimum value of the bias voltage changes over time. Therefore, in the LN modulator, ABC control (i.e. Auto Bias Control) as feedback control is performed to sequentially control the bias voltage to the optimum value.
In the ABC control, a pilot signal of a frequency of f0 is superimposed over the bias voltage. When the bias voltage has the optimum value, an optical signal output from the LN modulator does not have an f0 element, and, when the bias voltage shifts from the optimum value, an optical signal output from the LN modulator has the f0 element. Therefore, in the ABC control, the bias voltage is controlled such that this f0 element is minimum.
A related-art example is described, for example, in Japanese Laid-open Patent Publication No. 2008-092172
A drive signal applied to an LN modulator is generated by amplifying a data signal input in the LN modulator, and therefore an optical signal turned “ON” or “OFF” is output according to “0” and “1” of the data signal in the LN modulator. That is, in the LN modulator, ABC control is performed using the data signal input in the LN modulator.
Therefore, when an abnormality occurs in the data signal, the ABC control becomes unstable and therefore the bias voltage becomes indeterminate. As illustrated in FIG. 1, the modulation curve of the LN modulator is the cos-squared function curve in which the top turning point and the bottom turning point periodically repeat, and therefore there are a plurality of optimum values of the bias voltage. In the example of FIG. 1, there are three optimum values V1, V2 and V3. Therefore, if the ABC control continues in an unstable state while the data signal is in an abnormal state, the bias voltage may converge to the optimum value V3 due to an influence such as noise including the f0 element. Meanwhile, because of limitations of a circuit to supply a bias voltage to the LN modulator, there is an upper limit value of the bias voltage. In the example of FIG. 1, the upper limit value of the bias voltage is illustrated as Vmax.
Thus, since there is the upper limit value of the bias voltage, if the bias voltage converges to the optimum value near the upper limit value and a DC drift occurs, the bias voltage may exceed the upper limit value. In the example of FIG. 1, the optimum value V3 exceeds the upper limit value Vmax due to the DC drift.
If the bias voltage exceeds the upper limit value, it is not possible to supply an optimum bias voltage to the LN modulator, and therefore it is not possible to perform suitable modulation processing on a signal in the LN modulator.